Certain types of ceramic-based articles, such as engine exhaust system substrates and filters, are formed by the extrusion of a ceramic batch material. The extruded ceramic batch material constitutes an extrudate called a “log.” Upon exiting the extruder, the log has a high moisture content and so needs to be dried so that it can be further processed to form the final ceramic-based article.
One approach to drying logs involves using microwave power. This drying is usually accomplished by passing the log through a series of microwave dryers or “applicators.” Each applicator is set to supply what is believed to be the requisite microwave power to dry the log as the log passes through the applicator from an input side to an output side.
However, due to vagaries of the extrusion process, a pre-determined microwave-power drying profile can often result in substantial temperature variability over the log during the drying process, in particular at the start of the drying process when the power profile has not yet been optimized for the product being dried. Such temperature variability not only induces unwanted chemical reactions within the log but also adversely affects the log shape, which in turn adversely affects the ceramic article being manufactured. In particular, log-shape variations can cause the resulting ceramic article to fall outside of its shape tolerance limits.
Consequently, the process has to be manually adjusted using trial and error to get the drying process to reach a steady state where the temperature over the entirety of the log is close to a desired target temperature. This manual process can take anywhere from 30 minutes to 15 hours, depending primarily on the particular ceramic batch composition and the experience of the operators.